1. The Field of Invention
The present invention is in the field of upgrading hydrocarbon feedstocks that include a significant quantity of asphaltenes and hydrocarbons having a boiling point above 524° C. (975° F.) into lower boiling, higher quality materials.
2. The Relevant Technology
World demand for refined fossil fuels is ever-increasing and will inevitably outstrip the supply of high quality crude oil, whether as a result of actual shortages or due to the actions of oil cartels. In either case, as the price or shortage of crude oil increases there will be increasing demand for ways to better exploit lower quality feedstocks and extract fuel values therefrom. As more economical ways to process lower quality feedstocks become available, such feedstocks may possibly catch, or even surpass, higher quality crude oils as the primary source of refined fossil fuels for operating automobiles, trucks, farm equipment, aircraft, and other vehicles that rely on internal combustion.
Low quality feedstocks include relatively high quantities of hydrocarbons that have a boiling point of 524° C. (975° F.) or higher. They also contain relatively high concentrations of sulfur, nitrogen and metals. High boiling fractions typically have a high molecular weight and/or low hydrogen/carbon ratio and include complex compounds collectively referred to as “asphaltenes”. Asphaltenes are difficult to process and commonly cause fouling of conventional catalysts and hydroprocessing equipment.
Examples of lower quality feedstocks that contain relatively high concentrations of asphaltenes, sulfur, nitrogen and metals include heavy oil and oil sands bitumen, as well as bottom of the barrel and residuum left over from conventional refinery process (collectively “heavy oil”). The terms “bottom of the barrel” and “residuum” (or “resid”) typically refer to atmospheric tower bottoms, which have a boiling point of at least 343° C. (650° F.), or vacuum tower bottoms, which have a boiling point of at least 524° C. (975° F.). The terms “resid pitch”, “vacuum residue” and “vacuum reduced crude” (VRC) are commonly used to refer to fractions that have a boiling point of at least 524° C. (975° F.).
By way of comparison, Alberta light crude contains about 9% by volume vacuum residue, while Lloydminster heavy oil contains about 41% by volume vacuum residue, Cold Lake bitumen contains about 50% by volume vacuum residue, and Athabasca bitumen contains about 51% by volume vacuum residue. Resid contains even higher concentrations of fractions that boil at or above about 343° C. (650° F.), with vacuum tower bottoms almost exclusively comprising fractions that boil at or above about 524° C. (975° F.).
In conventional petroleum refining processes, crude oil typically is fractionated by an atmospheric distillation tower, producing fractions with different boiling points, including: gases, light naphtha, heavy naphtha, jet fuel, kerosene, diesel oil, atmospheric gas oil, and atmospheric bottoms (or atmospheric reduced crude). Among these products, gases undergo gas processing that eventually yields products including fuel, butanes, liquefied petroleum gas (LPG), and the like. The most commercially valuable fractions are the lower boiling liquid fractions, which undergo further hydroprocessing, including hydrocracking and hydrotreating, to yield gasoline blending products, jet fuel, kerosene, and diesel oil. The highest boiling fractions, atmospheric bottoms, are further fractionated by a vacuum distillation tower, producing fractions with increasing boiling points including: gas, light vacuum gas oil, heavy vacuum gas oil, vacuum residuum (or vacuum reduced crude), and asphalt. Light vacuum gas oil and heavy vacuum gas oil are further processed to yield gasoline blending products, while vacuum residuum is typically further processed by a coker, i.e., a system that reforms high boiling heavy oil (typically vacuum residuum) by thermal cracking, forming upgraded hydrocarbons and coke.
Coking is a thermal cracking process used in oil refineries to upgrade and convert petroleum residuum (bottoms from atmospheric and vacuum distillation of crude oil) into liquid and gas product streams, leaving behind a solid concentrated carbon material, petroleum coke. Coking products include gas, coker naphtha, coker gas oil, and petroleum coke, among which coker naphtha and coker gas oil are the more commercially valuable fractions, and can be further processed to yield kerosene, diesel, and gasoline blending products. According to the structure of petroleum coke, coke products include needle coke, sponge coke, shot coke, and anode grade coke.
Converting heavy oil into useful end products requires extensive processing, including reducing the boiling point of the heavy oil, increasing the hydrogen-to-carbon ratio, and removing impurities such as metals, sulfur, nitrogen and high carbon forming compounds. Examples of catalytic hydrocracking processes using conventional supported catalysts to upgrade atmospheric tower bottoms include slurry bed hydroprocessing that utilizes fine solid catalyst particles, fixed-bed hydroprocessing that utilizes a solid heterogeneous catalyst, ebullated- or expanded-bed hydroproces sing that utilizes a solid heterogeneous catalyst, and moving-bed hydroproces sing that utilizes a solid heterogeneous catalyst and is a version of fixed bed hydroproces sing. Non-catalytic processes used to upgrade vacuum tower bottoms include thermal cracking, such as residual oil coking (for which several commercial processes exist including delayed coking, fluid coking and ExxonMobil's proprietary Flexicoking® process), and solvent extraction. Solvent extraction is quite expensive and incapable of reducing the boiling point of the heavy oil. Conventional catalytic hydrocracking processes often involve rapid catalyst sintering, fouling, and deactivation and high catalyst cost, making them currently unsuitable for hydroprocessing vacuum tower bottoms unless substantially diluted with lower boiling fractions, such as atmospheric tower bottoms. Even with dilution of the lower boiling fractions, most existing ebullated bed processes operate at less than 65 wt % conversion, while most fixed bed processes have less than about 25 wt % conversion. Coking currently is the primary commercial method to upgrade vacuum reduced crude, but conventional coking processes typically are associated with low conversion rate, high risks of coking and fouling of equipment, extensive decoking time, and high decoking water and energy consumption.
When coke is formed in hydroprocessing processes other than coking, it tends to foul equipment and deactivate the reactor and catalyst, requiring extensive maintenance, expensive repairs and increased catalyst. Even when coke is formed in a coking process, it requires decoking using high pressure water and steam in a coking reactor, which consumes substantial time, energy, space, and water. Coking also tends to cause clogging and fouling within the coking system other than the coking reactor (e.g. furnace, resid feed line, filter, coking fractionator, and feed line).
Exacerbating the relatively low conversion levels using existing hydroprocessing systems is the inability to proportionally convert the asphaltene fraction at the same conversion level as the heavy oil as a whole. The result of disproportional conversion is progressive buildup of asphaltenes in the processed feedstock, with an attendant increase in the likelihood that coke and sediment will form in the reactor and other processing equipment. Apart from equipment fouling, coke and sediments can lead to instability of residual resid when used as a fuel oil.
In view of the foregoing, there is an ongoing but unsatisfied need to develop improved hydroproces sing methods and systems that can be used at a commercial level to upgrade heavy oil feedstocks. There is also an unsatisfied need to develop hydroprocessing methods and systems that can be used to upgrade vacuum tower bottoms and other low grade heavy oil feedstocks, increasing the production of upgraded liquid hydrocarbon products while reducing the formation of coke.